1. Field of the Invention
The present invention relates to a charging system for radio communication such as radio telephone and radio data transmission and, more particularly, to a charging method and system in a radio communications system using a DS-CDMA (Direct Sequencexe2x80x94Code Division Multiple Access) scheme.
2. Description of Related Art
In the DS-CDMA system, a plurality of channels are assigned to a single frequency band, and each user""s PN (pseudo-random noise) sequence or orthogonal code is generated to allow receivers to distinguish among different users signals. That is, the receiver performs reverse-spreading or despreading of the received signal by multiplying it by a replica of the PN sequence used at the transmitter. According to Interim Standard 95 (IS 95) released by TIA (Telecommunications Industry Association) in North America for Code Division Multiple Access (CDMA), the 64-bit Walsh orthogonal code sequence is used which consists of 64 kinds of code sequences with zero mutual correlation between them and can accommodate a maximum of 64 channels in one frequency band.
Since the receiver uses such a spreading code unique to the desired user signal, the signals coded with other users"" codes simply appear as noise, resulting in deteriorated quality of communications. Generally, it is necessary to limit the number of channels so as not to generate interference between the channels in the radio communications system.
In the above-described North American Standard TTA.IS-95, the electric field strength of a radio signal received from each user is measured at a base station and the received-signal strengths are controlled to become all the same at the receiving antenna of the base station. In other words, transmission power of terminal stations is precisely controlled to make the transmission power of a terminal station located far from the base station larger because of a relatively large loss in a propagation path to the terminal station. Contrarily, transmission power of a terminal station near the base station becomes smaller because of a relatively small loss of a propagation path to these terminal stations. In this way, the transmission power of each terminal must be precisely controlled in any DS-CDMA system serving mobile users.
Further, a terminal station also measures the electric field strength of a radio signal received from the base station, and reports a result of the measurement to the base station when necessary. In the case where the electric field strength of the received signal from the base station is not sufficiently large at the terminal station, this terminal station makes a request to the base station to increase the transmission power of the channel that is being used for communication. Then, the base station increases transmission power to the terminal station that has transmitted the request for power increase.
On the other hand, examples of conventional charging systems for portable telephones are disclosed in Japanese Patent Application Laid-open Publication No. 7-297954, No. 7-193533 and No. 7-283888.
According to Japanese Patent Application Laid-open Publication No. 7-297954, there are provided charging method and system for a case where a public portable telephone terminal station utilizes an access service to a public mobile telephone network through a private branch exchange (PBX) of a private area different from a public area.
Specifically, a charge for facility utilization of a private area is stored in a charge data base. Further a charge for service obtained by a public portable section (hereinafter to be referred to as a public PS) as well as communication charge via a public mobile telephone network are also stored in the charge data base. These stored communication charges and facility utilization charges are output to charge fees to the public PS. Alternately, information on the facility utilization charges received through a PBX and communication charges via the public mobile telephone network are stored in the charge data base of the public telephone network. The stored communication charges and facility utilization charges are output to charge fees to the public PS.
Based on this system, when the user of the public PS has obtained services via the PBX of the private area, it becomes possible for a PBX owner to charge the facility utilization fees to the user other than the utilization charges of the public mobile telephone network and for a public telephone network operator to charge the fees in place of the PBX owner.
According to Japanese Patent Application Laid-open Publication No. 7-193533, there are provided method and means for recovering (charging) a purchase cost of a booster by including it in a utilization charge of a portable telephone. Specifically, the booster has a controller having a charge information storing function therein. The controller uses the charge information storing function to store a registered telephone number of a portable telephone and communication time of the corresponding portable telephone to a memory. By reading the registered telephone number and the communication time thereof from the booster, the user who has purchased the booster and its telephone number can be identified. Therefore, the booster utilization fee for the portable telephone can be charged.
According to Japanese Patent Application Laid-open Publication No. 7-283888, there is provided a charging system for CDMA communications system. Specifically, speech information is encoded at a transmission rate depending on the amount of the voice information, and the encoded signal is transmitted at a transmission power depending on the transmission rate. A mobile terminal is provided with a charge setting means through which a user of the mobile terminal can set the maximum transmission rate. A communication charge is set depending on traffic of communication channels and the maximum transmission rate of voice information. Therefore, when increasing in traffic volume, a user will make effort to lower a maximum transmission rate of voice information as far as possible in order to minimize a communication charge. As a result, it becomes possible to lower interference between the channels during heavy-traffic time periods. It is also made possible to increase the number of communication channels depending on a reduction in interference.
As described before, in the CDMA communications system, communication channels are discriminated based on each user""s PN sequence or orthogonal code which allows a plurality of receivers to distinguish among different users signals in the same frequency band. Therefore, for each receiver, the signals coded with other users"" codes simply appear as noise.
To minimize such noise, in an up link from a mobile terminal to a base station, the base station controls the transmission power of each mobile terminal so that the same power is received from every mobile terminal and the power level is constant over time. By this arrangement, noise (interference) between up-link channels is minimized. In a down link from the base station to each terminal station, the received signal strength at the receiving antenna of the mobile terminal becomes the same if the transmission power for each down-link channel is set at the same value. In other words, when the transmission power of each down-link channel is set at the same level, noise (interference) between down-link channels is minimized.
When a mobile terminal receives insufficient electric field strength of a radio signal from the base station, this mobile terminal can make a request to the base station to increase the transmission power of the in-use channel. At the request of the mobile terminal, the base station increases the magnitude of the transmission power to that mobile terminal. Such a transmission power increase request may be generated when the sensitivity of signal reception is not sufficient. The receiving sensitivity may be reduced, for example, when a mobile terminal is located at a position relatively far from the base station or when the hardware of the mobile terminal develops a problem.
The increased transmission power of one channel for a certain mobile terminal causes other channels to increase in noise or interference, resulting in deteriorated quality of the other channels.
According to the above conventional charging methods, charge rates are basically determined depending on the time period during which one communication channel is occupied. According to Japanese Patent Application Laid-open Publication No. 7-283888, charge rates are determined based on both the transmission rate of voice information specified by the user and the channel-occupying time period. In other words, the charge rate is the same over all the channels regardless of an increase or decrease in the transmission power of the base station.
The inventor has found that such a situation implies an unfair aspect, in that the same charge rate is applied to a mobile terminal which generates noise for to other channels, and to other mobile terminals that are affected by the noise.
It is therefore an object of the present invention to provide a charging method and system that can eliminate the above-described unfair aspect.
It is another object of the present invention to provide a charging method and system that can inform a user of a change of charge rate.
According to a first aspect of the present invention, in a radio communications system where a basic station communicates with each terminal while controlling transmission power of a channel used for communication between the base station and the terminal, a magnitude of the transmission power is monitored and a communication charge rate is determined based on the magnitude of the transmission power. A communication charge for the communication is preferably calculated using as parameters the communication charge rate and a time period during which the communication charge rate is kept.
According to a second aspect of the present invention, at the base station, a magnitude of the transmission power is monitored and a communication charge rate is determined based on the magnitude of the transmission power. The communication charge rate is transmitted to the terminal. At the terminal, the communication charge rate is displayed on screen and/or a change of the communication charge rate is alerted by one of light and vibration.
According to a third aspect of the present invention, when the base station receives a request for changing the transmission power from the terminal, a magnitude of the transmission power is changed depending on the request and a communication charge rate is determined based on the magnitude of the transmission power. The request may be generated at the terminal in response to a user""s instruction.